Generally, the driven wheels of a vehicle start slipping when the drive force applied to the driven wheels exceeds the upper limit of the frictional force between the tires and the road surface when the vehicle is starting off or accelerating, and the slip ratio giving a measure of such slipping can be given by EQU .lambda.=(V.sub.W -V)/V.sub.W
where V.sub.W is the circumferential speed of the driven wheels, and V is the speed of the vehicle relative to the road surface.
The frictional force F between the tires and the road surface (the upper limit of the drive force that can be applied to the driven wheels) changes according to the slip ratio .lambda. as given in FIG. 9, and takes a maximum value when the slip ratio is equal to a certain value .lambda..sub.O. The frictional force F between the road surface and the tires is measured in the direction of the motion (the longitudinal direction) of the vehicle, but the lateral frictional force S (the side force) diminishes as the slip ratio .lambda. increases as indicated by the dotted curve in FIG. 9.
In view of this fact, it is necessary to detect the slip ratio .lambda. and control it as close to the prescribed value .lambda..sub.O as possible to maximize the longitudinal frictional force F between the tires and the road surface, and prevents the excessive reduction in the lateral frictional force S between the tires and the road surface and thus the occurrence of the side slipping of the vehicle.
Based on this recognition, a traction control system (TCS) has already been put into practical use wherein the speed of the non-driven wheels V.sub.V is assumed to be equal to the vehicle speed V, and the engine output is automatically curbed when the speed of the driven wheels V.sub.W becomes higher than the speed of the non-driven wheels V.sub.V, and the driven wheels are therefore considered to be slipping excessively. More specifically, as illustrated in FIG. 7, based on the speed of the non-driven wheels V.sub.V when the vehicle is accelerating, a first reference speed V.sub.R1 for determining the lower limit of the traction control, a second reference speed V.sub.R2 for determining the upper limit of the traction control, and a target speed V.sub.RP are derived by the following equations. EQU V.sub.R1 =K.sub.1 .multidot.V.sub.V +C.sub.1 EQU V.sub.R2 =K.sub.2 .multidot.V.sub.V +C.sub.2 EQU V.sub.RP =K.sub.P .multidot.V.sub.V +C.sub.P
where K.sub.1, K.sub.2, K.sub.P, C.sub.1, C.sub.2, and C.sub.P are constants, and K.sub.1 &lt;K.sub.P &lt;K.sub.2, C.sub.1 &lt;C.sub.P &lt;C.sub.2.
When the speed of the driven wheels V.sub.W has exceeded the second reference speed V.sub.R2, the engine output is controlled by limiting the supply of fuel to the engine or by delaying the ignition timing in the manner of a feedback control so that the speed of the driven wheels V.sub.W may be converged to the target speed V.sub.RP.
In such a conventional control process, the speed of the non-driven wheels V.sub.V (the vehicle speed V) for computing the reference values V.sub.R1, V.sub.R2 and V.sub.RP was obtained as an average value (V.sub.RL +V.sub.RR)/2 of the speeds of the right and left non-driven wheels V.sub.RL and V.sub.RR. The average value is used for the purpose of eliminating the influences of the difference between the speeds of the right and left wheels when the vehicle is making a turn (refer to Japanese patent laid-open publication No. 61-60331).
However, for instance, when a front-drive vehicle makes a turn at high speed (which may be considered as over-speeding for the given radius of curvature of the curve), the inner wheel (the inner one of the non-driven wheels) may be lifted from the road surface, and since a non-driven wheel lifted from the road surface receiving no rotating force from the road surface naturally has a substantially lower speed than the outer wheel, the average value of the speeds of the right and left non-driven wheels will give a substantially lower speed than the actual vehicle speed.
Since the reference values V.sub.R1, V.sub.R2 and V.sub.RP should be given as mathematical functions of the vehicle speed, if a vehicle speed lower than the actual vehicle speed is used for determining these reference values as illustrated in FIG. 8 (in which the reference value with the right and left wheels kept in contact with the road surface is indicated by the bold lines while the reference value with one of the wheels lifted from the road surface is indicated by the thin lines), the traction control may be started at time point t1 even though the actual point t2 for starting the traction control has not been reached, the engine output control may be started at time point t3 although the originally intended upper limit t4 has not been reached, the required degree of curbing the engine output may be by control amount B whereas it should be by control amount A as indicated at point t5, and the increase in the engine output may be only by control amount D and insufficient because the engine output should be actually increased by control amount C as indicated at point t6.